Information storage and retrieval systems, particularly those used in computer systems, typically record data magnetically, optically or magneto-optically onto several types of storage media, rotating magnetic or optic tape or disks for example. Such storage media may be utilized for document files, computer output memories, compact disk players, hard disk drives and the like. Depending on the type of storage media and recordation system, information may be recorded only once and then retrieved many times or it may be recorded, retrieved, erased and re-recorded indefinitely. Generally, the media of optical storage systems, such as compact disks, can only be written once and read many times. The limiting feature of tape recording systems is that the media deteriorates with use and time. Although the degeneration does not occur as quickly, magnetic disk media also wear and have a limited life span. While there are removable magnetic recording systems, typically, these systems are not suited for removability and long term reliability is a constant problem with such systems, due to the media's susceptibility to data corruption and erasure by outside stimuli, such as stray electro-magnetic fields. On the other hand, magneto-optical storage has the advantage of indefinite recording and erasure cycles without media deterioration problems and the media has the added advantage of removability and transportability between drives, as the media with its built in dust protection and non-contact operation is remarkably stable against normal outside influences.
Data stored on disks, whether magnetic, optical or magneto-optical, is contained within thousands of spiral or concentric tracks about the disk center. The total number of tracks and thus the storage capacity of the disk depends on the diameter of the disk utilized and the method of recordation of the disk. The amount of information that can be stored per unit area on the optical or magneto-optical media surface is much greater than the amount that can be stored on magnetic media, because the precision of an optical stylus is about 1 .mu.m, allowing the tracks to be spaced closely together. On the other hand, the track spacing of magnetic disks is limited to greater than 15 .mu.m, due to track runout and the signal to noise considerations of electromagnetic fields. Accordingly, the recording density of an optical or magneto-optical disk is between 10 and 100 times greater than that of a magnetic disk.
In both magnetic recording and magneto-optical recording, information is stored on a storage disk by orienting the magnetic field of the media at given points or bits along a given track. In order to record, access and read data on a disk, a transducer head (in the case of magnetic recording) or an optical head (in the case of magneto-optical recording, FIG. 1) is moved along a generally radial path across the surface of the storage disk as the storage disk is rotated. The generally radial movement of the transducer head in the case of magnetic recording or the optical head in the case of magneto-optical recording will either follow a straight line path or an arcuate path, depending upon whether a linear or rotary actuator is utilized to position the head.
The principles of magneto-optical storage are well known. Information is recorded on and erased from a thin film of magnetic material which is deposited on a substrate of suitable material. Information is encoded and stored in a sequence of magnetic bits oriented normal to the storage media surface in either of two possible orientations, north pole up or north pole down for example. To erase a track, all of its magnetized bits are oriented in one direction. Typically, for magneto-optical media, the magnetic or coercive force required to reverse a magnetic bit from, say, north pole up to north pole down, varies greatly with the temperature of the media. At room temperature, magneto-optical material is relatively resistant to changes in magnetization. The measure of this resistance is called coercivity. The coercivity of the material used in magneto-optical recording can be readily altered at a high temperature, called the Curie point. At the Curie point, about 150.degree. C., the coercive force necessary to reverse the magnetization decreases substantially and the magnetization may be reversed by a relatively small magnet or electromagnet.
Typically, magneto-optical storage devices comprise an optical head including lasers, collimating lenses, beam shaping prisms, beamsplitters, plane mirrors, an objective lens, a focus positioner, a tracking positioner, collecting lenses and detectors, as noted in prior art FIG. 1. These components are, as would be expected, complex, expensive and increase the mass and size of the optical head. Referring to prior art FIG. 1, a conventional magneto-optical head assembly 27 and its operation will now be described. During a recording operation, a laser diode 10 provides the heat source necessary for the storage media 20 to reach the Curie point. A 4 or 5 mm laser beam 29 provided by laser diode 10 passes through a grating 11, a lens 12, a polarizer 13 and a beamsplitter 14 to a movable reflecting mirror 15 and is focused at a point 28 (which represents 1 bit of information) on magnetic material 20 of a storage disk 18 by a movable objective lens 17.
In this manner, bit 28 on the storage disk 18 can be heated, thereby lowering the coercive force required to change the magnetization of the bit. A magnetic field will cause the orientation of the magnetic domain of bit 28 being heated by the laser beam 27 to be reversed. When the laser beam is turned off, the heated bit 28 on the storage disk 18 is cooled in the magnetic field generated by the coil 19, thereby freezing the bit in the desired orientation. The magnetic field generated in the coil 19 by a current is in one direction for writing and in the opposite direction for erasing.
Information is read from the magneto-optical storage disk using a laser beam of reduced intensity. The magnetic film's temperature increase and corresponding coercivity decrease produced by the reading laser beam are small enough that the direction of magnetization is not changed. Because of the magneto-optic phenomenon known as the Kerr effect, the polarization of a laser beam impinged upon the bit positions will be rotated as a function of the magnetic orientation of the bit. The polarization of laser beam portions reflected from bit positions on the disk is detected by opto-electronic detector circuitry, such as photodetectors. Signals from the detecting circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
For data retrieval, the laser beam emitted from the laser diode or other suitable light source 10 travels along a path between optical components 11-15 and 17, is reflected by the magnetic film 20 of the storage disk 18, passes through the lens 17 and is reflected by the mirror 15 to the beamsplitter 14 where some of the beam is passed through the wave plate 21 to the polarizing beamsplitter 22. Beams 29a and 29b upon leaving the polarizing beamsplitter then pass through cylindrical collecting lenses 23 and 24 and are then detected by photodetectors 25 and 26. A detected signal is then processed to extract the information contained therein.
A second function of the optical assembly is to derive tracking and focusing signals. Typically, prior art optical assemblies are provided with a focusing servo mechanism to detect and maintain the focus of the objective lens and a tracking servo mechanism for detecting a track and positioning the optical head such that the optical stylus is directed toward the desired track. Generally, in an optical assembly, the position of the optical stylus relative to a track is corrected using a feedback closed loop position circuit, which includes a two-stage actuator, namely a rough actuator and a fine actuator. The rough actuator is typically a linear actuator which moves the entire optical assembly radially across the optical disk from track to track. The fine actuation, on the other hand, is typically accomplished by rotating the mirror 15 with voice coils 16 and/or radially moving the optical lens 17 with voice coils.
Another known tracking method uses a rotary actuator to move the optical head from track to track. While rotary tracking in magnetic storage systems is superior to linear tracking, as the components are minimal and small allowing for a low profile arm with low inertia, the rotary tracking method used in magneto-optical assemblies has until now been inferior to magnetic rotary tracking systems for two reasons. First, the optical components are heavy, increasing the mass of the optical head, and consequently, the inertia. Second, the optical components are bulky, increasing the size of the optical head, and thus, magneto-optical heads are not capable of being used in multi-head, multi-disk drives. Moreover, if fine actuation is implemented in the typical magneto-optical rotary system, it typically involves a servo actuator for moving the objective lens radially across the tracks, which further increases the mass on the end of the actuator arm and the inertia of the arm.
One improvement in magneto-optical rotary actuators has been to decrease the mass of the arm by moving components off the arm, as described in U.S. Pat. No. 4,688,201 entitled "Focusing and Tracking Apparatus for an Optical Data Storage Device" by David Towner and David Campbell, issued Aug. 18, 1987 to the same assignee as the present application, which is incorporated herein by reference. Although Patent 4,688,201 reduces the weight of the arm, the arm and the overall storage device are still bulky and incapable of multi-head, multi-disks applications.
Focusing is generally accomplished using a closed-loop focus error circuit, which applies a current to a single-axis focus motor whenever a focus error is detected, thus actuating voice coils 30 that move the objective lens vertically to correct the focus of the stylus. The coils 30 react against a radial magnetic field formed by an opposed pair of cylindrical permanent magnets (not shown). This type of focus motor also adds to the weight of the typical optical head, further compounding the mass, bulk and inertia problems described above.
Consequently, the typical optical head measures several centimeters on a side, is 10 to 15 millimeters high, and weighs about 100 grams or more. Owing to its size, weight and the nature of the focus and tracking mechanisms, the standard optical disk assembly is inferior to magnetic disk assemblies in its track-seek time, it's use in multi-head, multi-surface, multi-disk drives, and its use in size critical applications such as lap-top computers and notebook computers. Furthermore, optical and magneto-optical assemblies also entail a complex assembly and adjustment process, as the reduced track spacing and resulting bit densities require greater tracking and focusing precision than magnetic storage systems. Due to the number of expensive components and corresponding complicated assembly, the average optical or magneto-optical storage assembly is much more costly than the average magnetic storage assembly.
Accordingly, there is need in the field of information storage systems for an optical or magneto-optical assembly, that has the customary advantages of stable media and large areal density, but that is also compact in size, light weight, less complex, and more economical. There is further need in the field for an improved optical storage assembly having a low moment of inertia in track-seek operations and having multi-surface, multi-disk capabilities. The present invention meets these and other needs.